Summary of Work: Age-related changes in muscle strength are an important contributor to frailty in the elderly. Our research has three main components. 1) Characterization of strength changes in the BLSA. 2) Comparison of exercise response to resistive strength training in young and old subjects through contract with the University of Maryland, College Park. 3) Examination of the motor unit and its relationship to muscle strength and exercise response. Concentric isokinetic strength was lower in older individuals, declining linearly from the 20s particularly in knee extensors. However, women showed smaller declines in eccentric strength with increasing age as compared to concentric strength. In men, the degree of change was similar to concentric strength with increasing age. With age a decline occurred in strength generated per kg of nonosseous fat free mass implying a changing ability of muscle to generate force with age. Analysis of longitudinal isometric arm strength found that strength per unit muscle mass may not decline to the extent observed in cross-sectional studies. The exercise training study is continuing. Most subjects are now in the final phase of the study which is whole body conditioning. A protocol was developed to examine motor unit function at different levels of muscle exertion in the quadriceps. Measurements in the Maryland contract are nearly completed, while the protocol has been started in the BLSA. A strong positive relationship occurred between the average motor unit size and the force generated. Furthermore, as force increases above 30% of maximum, the firing rate begins to rise. The changes are consistent with the Henneman principle i.e. the order of motor units activation during muscle contraction is from smallest to largest units. In analysis of multiple force levels from 68 subjects, the average amplitude of the motor unit and the firing rate accounted for three quarters of the variance in force. With fatiguing exercise, i.e. maintenance of 30% of maximum quadriceps strength for as long as possible, firing rates declined and amplitudes increased during the initial 2 minutes, and then varied by subject. The findings argue for unique strategies for maintenance of force during a fatiguing contraction.